This invention relates to a method of forming a metal alkoxide film on a substrate by a so-called sol-gel method using a metal alkoxide solution, and more particularly to the addition of a viscosity adjusting agent to the metal alkoxide solution.
It is well known to form a metal oxide film represented by titanium oxide film on a substrate such as a glass plate by a sol-gel method, i.e. by applying a metal alkoxide solution to the substrate, converting the alkoxide sol film on the substrate into a gel film by hydrolysis and then baking the gel film. Recently practical applications of this method have been increasing for forming metal oxide coating films for electronical, optical or ornamental purposes.
In forming a metal oxide film by the sol-gel method it is important to use a metal alkoxide solution having an adequate viscosity for both the productivity of the film forming process and the quality of the obtained film. In this regard U.S. Pat. No. 4,397,666 proposes to add a water soluble cellulose ether such as hydroxypropyl cellulose as a viscosity adjusting (increasing) agent to a silicon alkoxide solution. However, even though this proposal is employed it is not rarely that the obtained oxide film has minute cracks which are probably attributed to shrinkage of the gel film during baking.
By the sol-gel method it is not easy to form a relatively thick film of a metal oxide, viz. a film having a thickness of about 200-300 nm. The thickness of the obtained oxide film depends on the thickness of the initially formed sol film, and if an alkoxide solution having a very high viscosity is used in order to form a thick sol film it is difficult to obtain an oxide film of good quality. In this regard GB-A 2,208,874, which relates to the use of a chlorine containing titanium alkoxide to form a titanium oxide film by the sol-gel method, shows to repeat the steps of applying an alkoxide solution to a substrate, drying the alkoxide sol film on the substrate until a gel film having a desired thickness is formed. By this method it is possible to form a metal oxide film having a thickness of about 200-300 nm and fairly good quality, but the repetition of the coating and drying operations is unfavorable for the productivity of the film-forming process.